The application of fiber-reinforced thermoplastic granules for the production of work pieces in die casting procedures is sufficiently known from the prior art. Such granules, also called compounds, are produced in so-called compounding processes. Fiber strands are used for the production of such compounds or granules, consisting of multiple filaments. The fiber or yarn strands may consist, according to the prior art, of natural or cellulose fibers, or of glass fibers. From EP 1 436 130 B1, a method for the production of a granulate for the production of plastic molded components is known in this context, wherein, in a first step, continuous reclaimed cellulose fibers are coated with a matrix substance of a technical plastic and an adhesive agent in a pultrusion process, subsequently granulated, and then extruded again after subsequent drying process, and granulated again and then dried. This means that from this reference a method is known, with which one or more fiber strands are coated on their outer surface with a polymer, e.g. propylene, and said coated fiber strand is then processed to form a granulate in a first step. The coating with the polymer melt occurs, depending on the polymer used, at temperatures that in part are well over 200° Celsius. This means that already at this step the fibers are exposed to not insignificant thermal loads. A fiber strand produced in such a manner is then cut into granulates. It has been established that during the granulating procedure, i.e. during the cutting of the strand into individual sections, single fibers separate from the group, and extend above the individual granulates. This means that the fibers in the granulates are of quite different lengths. In another processing step, the cut granulates are fed into an extruder. In the extruder, a compound is produced through heat as well as mechanical effects, which should have a uniform distribution of the fibers in the mixture. Following the extrusion and molding to form an extrusion strand, the strand is again granulated in another step, and then processed as a material, in a die casting process, for example, to form work pieces.
It has, however, been determined in this connection, that the distribution of the fibers in finished work pieces is not uniform, i.e. it has been established that the work piece contains numerous so-called fiber clusters. The reason for this is quite clearly to be found in that the granulates produced in the first processing step actually exhibit, accordingly, a polymer layer on their outer surfaces. The coating with a polymer does not result, however, in a permeation of the fiber strand in the sense that also within the fiber strand, the polymer forms the matrix substance for the filaments. Regarding this, the processing of the granulate in the extruder clearly alters nothing. This means that the granulate according to the prior art has the disadvantage that the distribution of the fibers in the granulate, which ultimately serves for the production of the work piece, are not uniformly distributed, such that individual fiber clusters form in the work piece, resulting in a non-uniform consistency of a component produced thereby. Further, due to the introduction of heat in the two successive processing steps, this results in significant power consumption, firstly, and secondly, depending on the polymer used, and the associated melting temperature of said, in particular with polymers having a high melting point, such as polyamide, for example, when using cellulose fibers, there is the risk that said will be damaged. The cellulose fibers decompose in a manner dependent on the temperature, and the amount of time for which they are subjected to said thermal load.
Accordingly, the basic objective of the invention consists of providing a method for the production of a fiber strand wetted with a polymer, which enables the production of a granulate, having a homogenous distribution of filaments in the polymer matrix; furthermore, the thermal load during the production process should be kept small. For the following considerations, a fiber strand is defined as such having numerous filaments or fibers.